brood dude and Fred,
Don't worry about how much white vinegar to use, just add enough so that the dye bath has a good vinegar smell. If you really want a measured amount, somewhere between 1/4 and 1/2 cup of vinegar for 1 quart of water. But truthfully, you don't have to measure it because you can't put in so much that it will effect the dye job. The excess vinegar will simply go down the drain when you dump the dye bath and rinse the material to get rid of excess dye.
Poul,
Although I bought A.K.'s book on dyeing when it was first published a few years back, I was not impressed with it and felt I wasted my money. The new edition is just as poor in my opinion. It reads as if it were written by someone who has done very little dyeing, but because he is a prominent fly tyer, was asked to write a basic book on dyeing (much in the same vein as Art Sheck did with his book on building fly rods) since there was (and still isn't) a book devoted solely to dyeing fly tyeing materials on the market.
Heck, he doesn't even talk about using Synthrapol in his book and that is something that should be used by anyone dyeing fly tying materials. Synthrapol is a detergent, dye dispersant, and degreaser that was formulated specifically for dyeing in order to clean, degrease, and provide for even absorbtion of the dye to prevent splotchiness and uneven color retention.
A.K. also talks very highly of Veniard's dyes, which he calls "very powerful". Veniard's dyes are nothing more than acid dyes, but there is a problem with Veniard's dyes, almost none of their colors are composed of a single color or type of acid dye (yes, there are several different types of acid dyes and they set at different times when dyeing). They are made up of different colors of dye powder that are blended together. The problem with this is you cannot get the same shade the next time you use the dye because the powder might contain more or less of a color in the next 1/4 teaspoon of dye powder you use. Also, because they are also often made up of different types of acid dyes, if you use less or more of the dye powder, the color will be different, and that is not good if you want a little or more pastel shade of the same color because with Veniard's you will usually end up with a different color, not a lighter or darker version of what you think.
A.K. keeps talking about using RIT Dye, which is a very poor choice for fly tying materials since it is a so-called "union dye" that can be used on anything from cellulose-based (like cotton), poly, nylon, protein-based (like fly tying materials), and pretty much whatever other material you can think of. In other words, there is a lot of dye in a package of RIT that is useless for dyeing protein based materials like fly tying feathers and fur. RIT also doesn't produce good, bright, brilliant colors.
When I first started dyeing materials back in the late 1970's, the only dye I could readily get was RIT, and since Dave Whitlock had written about using RIT to dye deer body hair, I used it. I was never happy with the results. They were never the bright, clear, brilliant colors I wanted. Yes, I got some nice muted gold, yellow, orange, and green from using RIT, but it wasn't the deep, bright, brilliant color I was after.
I then tried Veniard's, which I got via mailorder. The colors were deep, bright, and brilliant and the material dyed in a fraction of the time it took with RIT. Unfortunately, when I tried to duplicate the color on another batch of the same materials, I couldn't get the same color despite having kept very detailed records of how much dye was used, the temp of the dye bath, how much vinegar was used, how long the material was in the dye bath. I then took a tying class from Dave Whitlock on tying his deer hair bass bugs and hoppers. During lunch, I asked him what I was doing wrong because I couldn't get Veniard's to give me consistent colors and RIT didn't give me the nice, bright, brilliant, clear colors he had in the deer body hair he had with him that he told us he dyed himself. Whitlock told me about Veniard's being blends of colors and why that kept me from being able to reproduce the colors with most of the Veniard's colors. He also told me about RIT being a union dye and what that meant. He finished by telling me he used acid dyes and that he got them from Orco Dye Company or Kiton. I've never looked back since that conversation of over 30 years ago.
So folks, please do yourself a favor and don't use RIT or Veniard's dyes.
The best how-to on dyeing I've seen is in Radencich's 1st book TYING THE CLASSIC SALMON FLY published in 1997. Ted Roebel, Ph.D. (I think that is how you spell his last name without looking it up in the book) wrote a superb chapter on dyeing and how it works for the book. Dr. Roebel is an organic chemist and I cannot recommend his chapter of this book or Radencich highly enough. Read this chapter and you will learn more from it that reading A.K. Best's book on dyeing 1,000 times.